Electromagnetic field producing apparatus and method for sequentially producing a plurality of fields

ABSTRACT

An apparatus and method for sequentially producing in a zone a plurality of electromagnetic fields. The lines of each field are produced to be generally curved and to have a direction at substantially every point in the zone substantially different from the direction of at least one other electromagnetic field line at that point. The apparatus comprises a plurality of field producing means each of which is responsive to applied electrical energy to produce an electromagnetic field. Means are provided for sequentially applying energy to the field producing means.

United States Patent Elder et al. Oct. 10, 1972 [54] ELECTROMAGNETIC FIELD [56] References Cited PRODUCING APPARATUS AND METHOD FOR SEQUENTIALLY UNITED STATES PATENTS PRODUCING A PLURALITY 0F FIELDS 3,457,502 7/1969 Cohn ..-.340/38 L Inventors: James T. Elder, Shoreview Village;

Donald A. Wright, Woodbury Village, both of Minn.

Minnesota Mining and Manufacturlng Company, St. Paul, Minn.

Filed: Dec. I7, 1969 Appl. No.: 885,874

Related US. Application Data I Continuation-in-part of Ser. No. 840,973, July 1 l, 1969, Pat. No. 3,665,449.

Assignee:

US. Cl. ..343/10l, 324/34, 340/38 L Int. Cl ..G0ls 1/02 Field of Search ..324/34, 41; 340/38 L, 258;

Primary Examiner-Carl D. Quarforth Assistant Examiner-J. M. Potenza Attorney-Kinney, Alexander, Sell, Steldt and Delahunt [57] ABSTRACT An apparatus and method for sequentially producing in a zone a plurality of electromagnetic fields. The lines Of each field are produced to be generally curved and to have a direction at substantially every point in the zone substantially different from the direction Of at least one other electromagnetic field line at that point. The apparatus comprises a plurality of field producing means each of which is responsive to applied electrical energy to produce an electromagnetic field. Means are provided for sequentially applyin energy to the field producing means. I

21 Claims, 12 Drawing Figures PATENTED 081 I 0 I972 sum 1 or 5 a I W in .F g f 2 0! 5 W u $9 .i wi hi 3 W m 0 A Tm 0 MW 7 C. 6 W i Z 5 T 4 l v z Q 5 .1 w 9 v w w w 2 p4 9 m a w J4M5 Z 1.05? I Y Qoxvflwfl 1 4 /6/17 MJ% '47 ORA/675' ELECTROMAGNETIC FIELD PRODUCING APPARATUS AND METHOD FOR SEQUENTIALLY PRODUCING A PLURALITY OF FIELDS CROSS REFERENCES This application is a continuation-in-part of our pending application, Method And Apparatus For Detecting At A Distance The Status And Identity Of Objects, U.S. Ser. No. 840,973, filed July 11, 1969, now US. Pat. No. 3,665,449.

BACKGROUND This invention relates in general to methods and apparatus for producing electromagnetic fields. More particularly, the invention relates to methods and apparatus for producing electromagnetic fields within a zone, such as an interrogation zone, into or through which a responder may be introduced or passed. When sufficient energy from the field is received by the responder element, the responder produces or alters its characteristic response. Typical responses include generation of a new signal and modulation and disturbance of the quiescence of the electromagnetic field.

Examples of systems which employ such apparatus and method are anti-pilferage systems and sortation systems. In a typical anti-pilferage system, each article to be protected against pilferage is provided with a responder and an electromagnetic field is produced in a zone through which the protected article would necessarily pass when being pilfered. An example of such a system is described in the above-identified pending application. In accordance with the system disclosed therein, when a responder such as the open strip, also discussed in the copending application, is passed into an applied magnetic field, and a major dimension of the open strip and a vector component of the magnetic field become oriented with each other, the magnetization of the open strip reverses at each alternation of the applied field. Each magnetization reversal produces a pulse of external polar magnetic field which is monitored in the vicinity of the interrogation zone, the presence of particular frequency components in the pulse being indicative of the presence of the marker. One possible sortation system application is sortation of passenger luggage at airport terminals. In such an application, each piece of luggage would be provided with a responder. Passage of the luggage and its responder through a field could conveniently be accomplished in many instances by producing the field proximate conveyor belts such as those now in use in many airport terminals.

Another possible application of the method and apparatus of the present invention would be a metal sensing system such as an eddy current detector system. In such systems, a metallic object such as a firearm acts as a responder. Upon'introduction of such a responder into the electromagnetic field of the system, the quiescence of the field is disturbed.

A difficult problem of the foregoing type responder systems is insuring that each responder which enters an interrogation zone receives sufficient energy. Although energy is a function of both time and intensity, to simplify discussion, when the term energy is used hereafter, it is only the intensity of the energy that is being referred to. The energy received by a responder at a point in a zone depends upon both the field intensity at that point and the orientation of the responder relative to the direction of the field at that point. The orientation of a responder is conveniently defined in terms of its geometry, usually the plane, axis, or length of the responder. Known responders include a planar receiving coil or sheet; an axial electric dipole; or a piece, usually an elongated strip, of isotropic magnetizable material having at least one long dimension. The orientation of such geometric characteristics of the responder relative to the direction of the field usually determines the proportion of the energy of the field at that point that is received by the responder. When the expression field direction or intensity is used herein, it is meant the direction or intensity of the field at a point; and, usually, this point is the point at which a responder is positioned. For a responder having just one such geometric characteristic, there is one orientation for which the responder receives the most energy. For each other orientation, a lesser amount of energy is received. The orientation for which the most energy is received shall hereafter be referred to as the orientation of maximum sensitivity. Usually, the orientation of maximum sensitivity of a receiving coil or sheet is that orientation in which the field direction is perpendicular to the plane of the coil or sheet; for an axial electric dipole it is that orientation in which the field direction is parallel to the dipole axis; and, for a magnetizable piece it is that orientation of the piece for which the field direction is parallel to the longest straight dimension of the piece. Hereafter, when reference is made to the orientation or direction of a responder, it is meant the direction of the geometric characteristic of the responder used to define the responders orientation of maximum sensitivity.

It will be appreciated that in some systems the direction of a responder in a zone may be random and may change as it is moved through the zone. This is true of anti-pilferage applications. In most such applications a responder can conceivably assume every possible direction and its direction can continuously change. In other applications such as those in which a responder is on an article carried by a conveyor belt, a responder is likely to, or may actually be constrained to assume fewer than every possible direction, and is not as likely to change its direction, as it passes through the zone. These considerations and the reliability desired of the system are important factors for determining the field requirements; that is, for determining whether a field component must be provided along every direction at every point in the zone, along every direction at some points in the zone, or along only certain directions at one or more points in the zone. By field component, it is of course meant a vector component the intensity of which provides sufficient energy to the responder.

The field requirements also depend on the responder geometry. They are of course less stringent for a multi-dimension than for a single-dimension responder. A multi-dimension responder is one which has two or more geometric characteristics each of which has a different direction. Examples of such multi-dimensional responders are L- or T-shaped magnetic strips such as those described in the above identified pending application. The previously mentioned flat coil, sheet, and electric dipole are single-dimension responders.

Reasons of cost and susceptibility to damage make single-dimension responders generally preferable to multidimension responders. In some applications, other considerations make single-dimension responders highly desirable if not essential. Concealability, which is a desirable feature of anti-pilferage applications, is one example of such a consideration.

' DISCUSSION OF PRIOR ART Several apparatusfor producing an electromagnetic field'are disclosed in French Pat. No. 763,681; One of those apparatus comprises a single figure-8 coil. The

.field of such a coil at a point has only one direction and has a total number of directions equal to only a portion of all possible directions. Another of the apparatus disclosed therein comprises a pair of coils energized simultaneously but out of phase to produce what has come to be referred to as a rotating field. Such a field provides at a point, a field having a different direction at different times, but these different directions still are only a portion of all possible directions. The size of such a rotating field is also small compared to the coil size. Of course by producing a field having a large, as opposed to a minimum, intensity the unreliability resulting from there being no field in some directions is reduced. Such a large intensity field would have more vector components of sufficient intensity than would a minimum intensity field. (A minimum intensity field is one which provides sufficient energy to a responder when the responder has an orientation of maximum sensitivity.) This effectively increases the directions provided by the field. Increasing the field intensity, however, usually also increases both the cost of the field producing means and the probability that the device will interfere with the use of other apparatus.

SUMMARY OF INVENTION We have discovered field producing apparatus and a method of energizing the apparatus which permits limitation of the applied field intensity to an amount not much greater than the minimum intensity. In one embodiment, the method and apparatus of our invention sequentially produce a plurality of individual fields, which together provide a field component along nearly every direction at virtually every point in a zone. In another embodiment, our invention sequentially produces only two electromagnetic fields. The field lines of these two fields are nearly everywhere in the zone orthogonal to each other. For a field intensity a factor of times greater than the minimum intensity, a field component of at least a minimum intensity exists along many directions at virtually every point in a zone. At any point defined by the crossing of two nearly orthogonal lines of fields of such an intensity, the fields would effectively provide a component along every direction in the plane containing the two fields. The apparatus of our invention also produces a relatively large zone.

For a particular application, the apparatus of our invention is designed to produce fields and the sequence period (the time required to energize each field producing means of an apparatus once) is selected such that, for an assumed responder velocity, the lines of each field are substantially unchanged in direction for a distance equal to the distance a responder would travel during one sequence period. In effect then, this distance becomes a point in the zone. We are able to make the sequence period very short and thus are able to employ fields the lines of which are generally curved.

Briefly, the apparatus for sequentially producing a plurality of electromagnetic fields within an interrogation comprises a plurality of electromagnetic field producing means. The fields will, when a responder having characteristics capable of being produced by the fields is present in said zone, produce the characteristic response of the responder. Each field producing means is positioned for producing in response to electrical energy supplied thereto an electromagnetic field the lines of which within a zone are generally curved. The direction of each field line at substantially every point in the zone is substantially different from the direction of at least one electromagnetic field line produced by another of the field producing means at that point. Means are provided for sequentially providing energy to each field producing means. The difference in the direction of the fields depends upon both the total directions required to be provided and the number of fields to be produced. When only two fields are to be produced and it is desired to provide every direction within a plane, to permit use of minimum intensity fields, the directions should differ by If more fields are produced, it may be possible to correspondingly reduce the difference between the directions of any two fields.

Each field producing means may comprise at least one long, straight conductor. If a means comprises more than one conductor, they are parallel to each other. The conductors of different field producing means are preferably either parallel or orthogonal to each other. When the conductors are orthogonal to each other, they preferably cross each other far from their respective ends.

We have found that a convenient way of obtaining parallel conductors is to use parallel segments of a generally rectangular loop comprising a coil which has been wound in either a generally 0 or a generally figure-8 shape. Hereafter, such coils shall simply. be referred to as 0 or figure-8 coils. Parallel segments of the coil are then employed as the long, straight parallel conductors. One embodiment constructed with such rectangular coils comprises at least one 0 coil and at least one figure-8 coil. The respective parallel segments of the coils are parallel, lie in essentially the same plane, and are spaced apart in that plane. When the coils are sequentially energized, they have been found to provide fields having directions which nearly everywhere within a relatively large zone are substantially orthogonal to each other.

For critical applications, those requiring a high degree of assurance of producing or altering the characteristic response of a responder, a preferred embodiment of the apparatus of our invention comprises three sequentially energized electromagnetic field producing means. Each produces within a zone an electromagnetic field the lines of which within the zone are generally curved. At virtually every point within the zone, the directions of the lines of the three fields are substantially mutually perpendicular. Such an apparatus shall hereafter be referred to as a threedimensional field producing apparatus; and, an apparatus which produces two fields the lines of which are substantially orthogonal to each other at substantially every point in the zone shall be referred to as a two-dimensional field producing apparatus.

In a preferred embodiment, a three-dimensional field producing apparatus comprises a pair of figure-8 coils and an 0 coil. The parallel end and center segments of a first figure-8 coil act as long conductors to produce a first field. Only the field produced by the center segment of the second figure-8 coil is required as the second field. Each of the coils is conveniently generally planar and the coils lie in closely spaced substantially parallel planes. The two figure-8" coils are orthogonal to each other and their center segments intersect each other near their mid-points. The 0 coil parallel segments which provide the third field are parallel to the end and center segments of the first figure-8 coil. Each of the 0 coil parallel segments lies between the center and one end segment of the first figure-8 coil.

Another embodiment of a three-dimensional field producing apparatus comprises a pair of ferromagnetic linear electromagnets orthogonal to each other and crossing each other at approximately their mid-points in combination with an air core loop the periphery of which passes proximate the magnetic poles of the linear electromagnets.

We have sequentially energized the field producing means, i.e., we have produced pulsed fields, by switching a sinusoidally varying energy source from one means to another. The field produced by each means during one continuous energization is hereinafter referred to as a pulse of field or a field pulse. Also for those apparatus in which each field producing means includes a coil, we have produced each field pulse by discharging an underdamped inductive capacitive (LC) circuit. The coil of each field producing means is the significant inductive component of the circuit. A capacitor is provided in series with the coil to provide the capacitive component. Means are provided for sequentially supplying electrical energy to each LC circuit. The discharge of this energy by the LC circuit produces an underdamped sine-wave field pulse. In each of the foregoing cases, a pulse sequence consists of one field pulse from each field producing means and each field pulse comprises a sinusoidal-like field the direction of which reverses or alternates several times. It is to be appreciated, however, that by forming an overdamped LC circuit in a similar manner, the direction of a pulse would not alternate but would be a single uni-directional pulsation for each discharge of the overdamped circuit.

It should be noted that a sequence period need not be equal to the sum of each of the pulse field durations. Indeed, we have found in some cases that it is desirable to include in each sequence period a continuous interval at least one second long during which no field is produced. By providing such a one second interval we find that the operation of a heart beat timing control device, commonly referred to as a heart pacemaker, is virtually unaffected by the fields of our apparatus.

BRIEF DESCRIPTION OF THE DRAWING The principle of the invention and the relative size and arrangement of the different field producing means of various exemplary embodiments of the invention will be better understood from the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is an end view of two long, straight parallel conductors and flux lines produced in response to a current flowing in the conductors;

FIG. 2 is an end view of four long, straight parallel conductors and the flux lines produced in response to a current flowing in the conductors;

FIG. 3, View A, is a schematic plan view of a pair of generally rectangular o shaped coils; View B is a perspective of View A illustrating a zone within which the field lines of the coils are nearly orthogonal;

FIG. 4, View A, is a composite plan view of a grid formed by superimposition of three coils consisting of a pair of generally rectangular figure-8 coils and one generally rectangular 0 coil; Views B, C and D are individual schematic plan views of the coils of View A;

FIG. 5, View A, is a side schematic plan view of two of the coils of FIG. 4 and a cross-section of a zone within which vector representations of the fields produced by the horizontal segments of the coils of FIG. 4 are shown; View B is a top schematic plan view of the coil of FIG. 4 not shown in View A, and a crosssection of a zone within which vector representations of the field produced by the vertical center segment ofa figure-8 coil of FIG. 4 are shown;

FIG. 6 is a front schematic plan view of a threedimensional field producing apparatus comprising a pair of ferro-magnetic linear electromagnets and an air core loop;

FIG. 7 is a circuit schematic illustration of the windings of an electromagnet of FIG. 6;

FIGS. 8 and 9 show representations of magnetic fields produced by the applied field producing apparatus of FIG. 6;

FIG. 10, View A, illustrates two sets of magnetic field lines of the applied field producing apparatus of FIG. 6; Views B and C are vector representations of the direction of each of the three fields produced by the apparatus of FIG. 6 at an intersection of each of the two sets of lines of View A;

FIG. 11 is a schematic of a circuit for sequentially energizing a plurality of electromagnetic field producing means;

FIG. 12 is a schematic of another circuit for sequentially energizing a plurality of electromagnetic field producing means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an end view of two long, straight, parallel conductors 12 and 14 showing the flux lines of the magnetic fields produced by a current flowing in each conductor at different intervals of time. Circular line 13 is one member of a set of concentric circular flux lines produced by a current flow in conductor 12; line 15 is a similar flux line of the set produced by a current flow in conductor 14. Other lines defining the electric field, which also characterize an electromagnetic field produced when a current flows in a conductor, are related to magnetic flux lines such as the set including lines 13 and 15 in a well known manner. Thus, for simplicity of illustration only the magnetic flux lines have been shown in FIG. 1. For similar reasons, only the magnetic properties of the electromagnetic fields will hereafter be discussed and illustrated.

As shown, the lines of the fields of each of conductors 12 and 14 are curved yet substantially orthogonal to each other at their points of crossing throughout the area 10 enclosed within dashed line 17. Area 10 represents a cross-section of an interrogation zone and thus is illustrated as rectangular because this is often the shape of the cross-section of a passageway in which an interrogation zone of an electromagnetic field is to be produced. Doorways, hallways or other similar passageways are typical of such areas.

Assume that dimension 19 of area 10 is the width of a doorway and that conductors l2 and 14 are vertically oriented. Dimensions 19 and 21 thus correspond to the width and length respectively of the interrogation zone. The considerations for determining the actual length of a zone were previously discussed. The intensity of a field is known to vary directly as the amplitude of the current flowing in the conductor and inversely with respect to distance from the conductor. For a specified application having a width 19, having the conductors l2 and 14 spaced apart distance 23 and spaced from the edge of the passageway distance 25, and for a responder requiring a particular sufficient energy, it can be determined by vector analysis of each of the fields at their points of crossing at the farthest point in the area (near dimension 21) what amplitude of current flow in conductors 12 and 14 is required.

FIG. 2 is an end view of four long, straight parallel conductors l6, 18, and 22 showing two sets of lines of magnetic fields which could be produced by current flowing in the conductors. One set of flux lines is shown in solid lines, the other in short dashed lines. The solid lines are those which would be produced if conductors 16 and 18 were simultaneously carrying like amounts of current in opposite directions with no current flowing in conductors 20 and 22; the short dashed lines are the field for a converse current flow, i.e., no flow in conductors l6 and 18 and a flow of like amounts but in opposite directions in conductors 20 and 22. It should be noted that for equal dimensions 23 of FIGS. 1 and 2 an area 10 of much longer length 21 can be produced in the same amount of time (in both cases, only two F field pulses need be produced). The analysis for determining the current requirements for the conductors of FIG. 2 is generally the same as that described with reference to FIG. 1. It will also be appreciated that by providing another set of such an arrangement of conductors and orienting these two sets in closely spaced parallel planes with the conductors of the respective sets orthogonal to each other, a three-dimensional apparatus will be provided. With such an apparatus there will be at substantially every point within the area 10 at least once during each pulse sequence of four pulse fields each of three nearly mutually perpendicular magnetic fields. The end conductor of such another set is indicated in long-dashed lines in FIG. 2 and identified as 27.

In- FIG. 3 there is shown a two-dimensional field producing apparatus. Referring now to FIG. 3, View A, the apparatus is shown to comprise a pair of o coils 24 and 26. For purposes of analysis, the vertical segments of the coils can be considered as long, straight parallel conductors. The coils 24 and 26 lie in substantially the same plane but are displaced such that adjacent vertical segments are of different coils and such that the horizontal separation between the inside vertical segments, segments 28 and 30, is much less than the horizontal separation of vertical segments of the same coil. In this way, the lines of the fields between the inside vertical segments are approximately those shown in FIG. 1. View B illustrates a zone of such unequal spaced coils having a length 21, width 19 and height '34. If the coils were equal spaced, i.e., if the separation between the inside segments was equal to one-half the separation of segments of the same coil, the fields produced would have lines like those of FIG. 2. We have found that a combination of such equal-spaced coils having 3% foot long vertical segments, 4 foot long horizontal segments and comprising ten turns of stranded No. 10 A.W.G. wire (such as Belden No. 30610) will produce a substantially two-dimensional field in an interrogation zone the volume of which has dimensions of about three feet high by 3 feet wide by 5 feet long when pulsed by an underdamped sinusoidal current of an initial peak amplitude of 180 amperes.

Referring now to FIG. 4 there is shown a threedimensional field producing apparatus. In View A, a schematic plan view, the apparatus appears as a grid 39. This grid is a composite of three coils. In a preferred embodiment, these coils lie in closely spaced, parallel planes. Each of the coils is individually shown in one of Views B, C and D. The grid is indicated as having three vertical components 40, 42 and 44 and five horizontal components 46, 48, 50, 52 and 54. As will become apparent following a discussion of the individual components, each of these'components comprises at least one segment of a coil; most comprise coincident segments of several coils. It should be noted that the fields produced by the coil segments forming components 40 and 44 are not required. They will be produced, however, and do, if the components 40 and 44 are sufficiently close to component 42, reduce the length" of the zone. Also, in those cases where a component comprises a segment of more than one coil, only the field of one segment is required. Therefore, in the following discussion of the individual coils, only those fields which are required shall be discussed. Referring now to View B, there is shown an up-right generally rectangular figure-8 coil 55. The coil comprises a conductor wound in a figure-8 fashion to form two generally rectangular series connected portions of nearly equal size. The portions are indicated generally as 58 and 60; each has substantially parallel top and bottom lengths. Portion 58 is shown to have a top length 72 and a bottom length 74 and portion 60 is shown to have a top length 76 and a bottom length 78. The bottom length 74 of loop 58 and the top length 76 of loop 60 are coincident to form the center segment of the figure-8 coil 55. The other top and bottom lengths of portions 58 and 60, top length 72 and bottom length 78, form the end segments of the figure-8 coil 55. Terminals 80 and 82 permit coupling of a pulse of energy to the figure-8 coil. The arrows within the two loops, one of which is indicated as 84, indicate the direction of current flow through the various segments of the loop for a current of one polarity. It is apparent from View B that length 72 corresponds to horizontal component 46, that lengths 74 and 76 correspond to horizontal component 50 and that length 78 corresponds to horizontal component 54. Referring now to View C there is shown an coil 57 the horizontal segments of which are indicated as 86 and 88 and which respectively correspond to horizontal components 48 and 52 of grid 39 (View A). View D illustrates a figure- 8 coil turned on its side. The lengths of the coil identified as 94 and 96 correspond to vertical component 42 of grid 38 (View A) and lengths 90 and 92 correspond to components 40 and 44 respectively.

Vector representations of the magnetic force lines of the fields produced by the three field producing means (coils 55, S7 and 59) of FIG. 4 are illustrated in FIG. 5. View A is a view of field vectors in a plane both parallel to the coil 59 vertical lengths (e.g., lengths 94 and 96) and normal to the plane of the coils 55 and 57 showing an end view of their respective horizontal lengths 72, 74, 76, 78, 86, and 88. Coil 59 is not shown. An area of a cross-section is drawn in short dashed lines and indicated as 39. Field vectors are illustrated at three arbitrarily selected points 110, 112 and 114. The vectors identified as the a vectors correspond to the field produced by the lengths 72, 74, 76 and 78 of coil 55; the vectors identified by the character b correspond to the field produced by the conductors 86 and 88 of coil 57. The field vectors of the field produced by the third coil, coil 59, at points 110, l 12 and 114 are illustrated in View B wherein the field is viewed in a plane both parallel to the horizontal lengths of the coils and normal to the plane of the coils. Coils 55 and 57 are not shown. Area is again shown by dashed lines and the vectors of the field of coil 59 are identified by the lower case character c. In View B, an end view of the center lengths 94 and 96 and the end lengths 90 and 92 of coil 59 is shown. The dimension 21 of area 10 extends only a part of the way from the pair of center segment lengths 94 and 96 to the end conductors 90 and 92. This is because end conductors 90 and 92 carry current in a direction opposite to that carried by conductors 94 and 96. Conductors 94 and 96 carry current in the same direction and can be treated as a single conductor. It will be appreciated that in such a figure-8 coil, the separation between the center segment conductors 94 and 96 and end conductors 90 and 92 can be controlled to control dimension 2]. The field vector diagrams of View A of FIG. 5 illustrate that the coils 55 and 57 produce two-dimensional fields at points 110, 112 and 114 and those of View B illustrate that the field of coil 59 adds a third dimension to the fields.

In an embodiment of the apparatus of FIG. 4 specifically intended for use in conjunction with an anti-pilferage system for detecting markers such as those described in the afore-identified pending application, and wherein the interrogation zone was an area leading to a doorway, an interrogation zone of at least 5 feet high by 2 feet wide by 2% feet long (dimensions 34, 19 and 21) was produced. The grid 39 formed by coils 55, 57 and 59 had the following dimensions: vertical components 40, 42 and 44 were 6 feet long and horizontal components 46, 48, 50, 52 and 54 were 4 feet long. The 0 coil was centered about the intersection of the center segments of the figure-8 coils and had its lengths 86 and 88 spaced 2] inches respectively above and below lengths 74 and 76 of coil 55. Coils 55, 57 and 59 all were made of No. 10 vinyl-covered stranded wire (Belden No. 30610). Coil 55 comprised 6 turns, coil 57 comprised 10 turns, and coil 59 comprised 7 turns.

It should be noted that the planar construction of FIG. 4 provides a structure which is inexpensive to produce and lends itself to packaging in a variety of aesthetic forms. Such a planar structure may be easily installed along either side of a passageway either leading to a doorway or an otherwise channeled traffic pattern. Another advantage of the structure is that the coils can easily be balanced resulting in little or no mutual inductance between an energized coil and the other two unenergized coils. In this way, virtually all of the energy produced by a coil is available for production of a field within the interrogation zone.

In FIG. 6, there is shown another three-dimensional field producing apparatus. An apparatus 100 is shown to comprise a pair of ferro-magnetic electromagnets 102 and 104, orthogonal and crossing each other at approximately their mid-points. The apparatus further comprises an air core loop 106, the periphery of which passes proximate the magnetic poles of electromagnets 102 and 104. In an embodiment of the apparatus of FIG. 6 suitable for producing an interrogation zone in a space such as a doorway, an apparatus 100 is placed on one or both sides of the doorway. For such an embodiment the electromagnets 102 and 104 may each be formed of solenoids surrounding a 5.1 centimeter square bar of iron. The bar is conveniently a laminate formed of 0.0457-centimeter thick by l42-centimeter long by 5 .l centimeter wide sheets of transformer steel, type M-1 9. Each solenoid comprises a pair of l25-turn windings of 0.205-centimeter diameter enameled wire distributed uniformly along the length of the bar and an additional pair of 60-turn windings of 0.259 centimeter diameter enameled wire at each end of the bar. Both additional 60-turn windings are uniformly wound in a bifilar-aiding fashion with the end-most 60 turns of the corresponding l25-turn winding. These windings are schematically shown in FIG. 7 where the 60-turn windings are shown as 108 and 110, and the l25-turn windings are shown as 112 and 114. The 60-turn windings are shown wound in series with each other and in series with the parallel combination of the 125- turn windings. This combination of windings shall hereafter be referred to as the coil winding and the terminal shown as 116 shall be referred to as the coil input terminal and the terminal designated 118 shall be referred to as the coil output terminal. For a construction such as that described and illustrated in FIGS. 6 and 7, we have found that the poles of the linear electromagnets are located approximately 7.5 centimeters in from the ends of the steel bars. The air core loop 106 has a diameter equal to the pole separation of electromagnets 102 and 104. The loop was made of turns of enameled wire 0.205-centimeter in diameter. A circuit for sequentially switching a source of energy of 60 hz. sinusoidal waveform among each of the aforedescribed coils is disclosed in the afore-identified pending application.

In another embodiment of an apparatus for providing a a plurality of fields alternating at a frequency of about 800-1 ,000 hz., electromagnets 102 and 104 each comprised a l42-centimeter long, 5.1-centimeter thick and 5.1-centimeter wide lamination of short pieces of transformer steel about 0.0457 centimeters thick, 5.1 centimeters wide and not more than 60 centimeters long. The pieces were adhered to form the lamination with an approximately 50 micron thick layer of viscoelastic transfer adhesive such as 3M Company No. 467. For this embodiment, each of electromagnets 102 and 104 were provided with one 18- turn winding about 2 feet long approximately centered along the length of the laminate. These windings and the winding of air core loop 106 each were of a vinyl insulated number A.W.G. stranded wire commercially available as Belden No. 30610. The air core loop comprised ten turns. We found that the vinyl insulation, short pieces, stranded wire, and viscoelastic adhesive each reduced compared to the corresponding materials used in the previously described embodiment) the accoustical noise generated by the apparatus.

FIGS. 8 and 9 respectively illustrate free space characteristic magnetic fields of an electromagnet and an air core loop such as those of FIG. 5. (The figures illustrate the magnetic field lines lying in the plane of the paper.) As shown, the field of the vertically oriented electromagnet includes components which are vertical (parallel to the y axis) and also includes significant horizontal, x axis, components in the regions near its poles. Similarly, the magnetic field components of the air core loop include components parallel to the x axis and components parallel to the y axis.

FIG. 10, View A, illustrates two sets of magnetic field lines from each of means 102, 104 and 106 of an apparatus 100; the force vectors of these lines at their points of intersection are shown in Views B and C. For the sake of clarity, the lines representing the field produced by means 102 are shown uniformly dashed;

' the lines representing the field produced by means 104 netic field strengths is shown at Views B and C of FIG.

10 for points m and 11 respectively. Components r, s, t of View B represent the magnetic field strength of the fields at point m respectively produced by means 104, 106 and 102; components r', s, t of View C are corresponding magnetic field strength vectors of the fields at point n. As shown, the components of each set are substantially mutually perpendicular even though the r, s, t component directions are different from the r', s, t directions. Inspection of FIG. 10 makes it readily apparent that the vector components of intersections of other sets of lines will be nearly perpendicular, too.

A circuit for sequentially impressing electrical energy on each of a plurality of field producing means which each have a coil such as the coils of either FIG. 4 or FIG. 6 is shown in FIG. 11. The circuit shown is particularly useful for applications requiring a field having a very high signal to noise ratio. An example of such an application is an antipilferage system employing a responder (or marker) which produces a very low intensity signal. The circuit is shown to comprise an energy source section, shown generally as 170, three energy transfer sections 172A, 172B and 172C and a sequence-control signal generator 183. The circuit is not limited to having only or exactly three transfer sections 172; three are shown to facilitate description of the circuit relative to the fields producing apparatus including three coils such as those of either FIG. 4 or FIG. 6. Only one of transfer sections 172 is shown in detail as they all may be identical. Obviously, if the electrical properties of the coils are not identical, e.g., if they had different inductive properties, and if it is desired to transfer to each coil energy identical in duration and alternation frequency, some component values of the sections would differ in a well known manner. Conveniently, source section may comprise a source of A-C voltage 174 such as ordinary 1 17 volt line voltage and a conventional voltage doubler rectifier 176. The voltage doubler 176 provides a D-C output potential of approximately zero and minus 200 volts, on leads 178 and 180 respectively, to each of energy transfer sections 172A, 1728 and 172C.

Energy transfer section 172 includes a switch, or,

more specifically, a transistor 182A which is actuated or rendered conductive when a control signal from a sequence-control signal generator 183 is impressed on node 184A. When transistor 182A conducts, energy transfer section 172A is enabled to pass a pulse of energy to an associated field producing means coil. In FIG. 11, the coil associated with energy transfer section 172A is schematically shown as an inductor 186. A capacitor 188 stores a pulse of energy from the energy source and in response to conduction of transistor 182 passes the pulse to the field producing means as a damped pulse of energy. The circuit for charging capacitor 188 and subsequently causing transfer of this charge to field producing means or inductor 186 is .believed to be unique and comprises inductor 190, a silicon controlled rectifier (SCR) 192, and a circuit, shown generally as 194, for triggering SCR192. This circuit is the subject of a copending patent application of Donald A. Wright, one of the co-inventors of this application. Trigger circuit 194 comprises normally non-- conducting transistor 196 the base lead of which is coupled to one end of each of resistors 198 and 200. The other end of resistor 198 is also coupled in series with a blocking capacitor 204 and diode 206 to the collector of transistor 182A. One plate of capacitor 204 and the cathode of diode 206 are also common to one lead of a resistor 208 the other lead of which is connected to the zero volt reference lead 178. Normally, with transistor 182 in a non-conducting state, the potential across capacitor 204 is about 200 volts and thus current to the base of transistor 196 is cut off, holding the transistor in its normally non-conducting state. When transistor 182 conducts, capacitor 204 charges toward 212 volts and the charging current through capacitor 204 turns on transistor 196 until the potential across capacitor 204 reaches 212 volts. The remainder of trigger circuit 194 comprises the emitter and collector resistors, 210 and 212 respectively, of transistor 196, a capacitor charging network formed by resistor 214 and zener diode 216 and a capacitor 218. The gate lead of SCRl92, is common to the emitter of transistor 196 and its emitter resistor 210 so that, with both the other end of resistor 210 and the cathode lead of SCRl92 held at approximately minus 200 volts, the SCR will be rendered conductive or triggered only whenever transistor 196 conducts. Transistor 196 conducts when a pulse applied to node 184 forward biases transistor 182. Current flows from the emitter to collector of transistor 182, through diode 206, capacitor 204 and resistor 198 to turn on transistor 196. With transistor 196 on, capacitor 218 discharges through resistor 212 and transistor 196 to provide a pulse of current to the gate of SCR192 thereby triggering the SCR into conduction. Upon conduction of SCR192, current flows through capacitor 188, inductor 190 and SCR192 to begin storage of energy in capacitor 188. To insure a sufficiently rapid storage, it has been found necessary to select the value of inductor 190 to be not more than about one-fifth that of inductor 186. In this way most of the current from capacitor 188 flows through inductor 190 rather than through inductor 186. When capacitor 188 is charged to approximately 200 volts the current in inductor 190 is at a maximum. This current continues to flow until the energy in inductor 190 is dissipated, and as a result capacitor 188 charges to a voltage of about --300 volts. This final voltage depends somewhat on the ratio of the inductances of inductors 186 and 190, and also on resistive losses in inductor 190 and other components through which the current passes. When the voltage across capacitor 188 has reached its peak and current has stopped flowing in inductor 190, SCR192 stops conducting. Capacitor 188 now discharges into field-producing inductor 186, generating a characteristic damped sinusoid waveform of current within the coil. A resistor 191 of very small resistance is provided between one end of inductor 186 and the zero volt reference lead 178. The voltage developed across resistor 191 is exactly proportional to the current in inductor 186 and may thus conveniently be employed as a synchronizing signal for a detector system. In such a system, a detector indicates the presence of a responder upon detection of a characteristic signal at a particular time specified relative to the time base of the electromagnetic field.

For an application not requiring as great a signal to noise ratio as the afore-described anti-pilferage system, the circuit of FIG. 11 could be greatly simplified. Each field producing means could be provided with its own capacitor and a single supply voltage coupled to each of the capacitors. FIG. 12 is a circuit diagram illustrative of the basic components of such a system. Referring now to FIG. 12, DC supply voltage 230 is adapted to be coupled to each of capacitors 232, 234 and 236 through, respectively, switches 238, 240 and 242. These switches are each controlled by a pulse sequence-control device not shown but which would perform the same function as the like device previously discussed. Each switch is adapted to switch an associated capacitor into circuit with either the D-C voltage or its associated coil. The coils associated with switches 238, 240 and 242 are shown respectively as inductors 244, 246 and 248.

The function of sequence-control signal generator 183 is to provide a control signal to each of nodes 184A, 184B and 184C in sequence to produce a field pulse from each of the coils associated with energy sections 172A, 172B and 172C such as the coil schematically illustrated as inductor 186. Sequence-control signal generator 183 could conveniently comprise a plurality of monostahlc multivibrators coupled in a ring such that switching of each multivibrator from its unstable to its stable state effects a converse switching of states of the succeeding multivibrator in the ring. The individual multivibrator outputs would be provided to nodes 184. Another way of providing control signals to nodes 184 would be by connection of individual stages of a simple flip-flop counter driven by a free-running multivibrator.

Components for an embodiment of the circuit shown in schematic in FIG. 11 and which has been used at different times to sequentially energize both the apparatus of FIG. 4 and the IS-turn apparatus of FIG. 6 are given below in Table I.

capacitor 25 microfarad, 230 diode D1 silicon rectifier, C1 VAC 1 amp, 600 volt diode silicon rectifier, resistor I I5 ohm 2 watt D2 1 amp, 600 volt R1 Resistor [000 ohm [00 watt capacitor 4500 microfarad, R2 C2 250 volt, electrolytic resistors I000 ohm watt transis- PNP Transistor R4A, R48 R4C ors 182A, 2N5l39 182B, 182C diode 206 silicon diode lN9l4 capacitor .l microfarad,

204 400 volts resistor 470 ohms /4 watt resistor 220 ohms V4 watt R198 R200 resistor 1000 ohms watt resistor l megohm V4 watt R208 R214 resistor 47 ohms 1% watt transis- NPN Transistor R212 tor 196 2N34l4 resistor ohms watt zener Zener diode, R210 diode 216 400 mw, l8 volts capacitor .22 microfarad, capacitor 80 microfarad, C218 I00 volts C188 230 VAC resistor 47 ohms 1 watt capacitor .22 microfarad, R3 wirewound 600 volt resistor .02 ohm 10 watt inductor Field Producing R191 wirewound 186 Inductor; 300

microhenry inductor 60 microhenry; silicon SCR 2N3898 190 formed of 8 turns of control- A.W.G. No. 10 stranded wire led recti While the invention has been described in certain preferred forms and particular areas of the invention have been illustrated other modifications and uses of the invention will be readily suggested to others with the foregoing discussion before them.

What is claimed is:

I. An apparatus for sequentially producing a plurality of electromagnetic fields within an interrogation zone comprising:

a plurality of field producing means, each being positioned for producing in response to electrical energy supplied thereto an electromagnetic field the lines of which in said interrogation zone are generally curved and have a direction at substantially every point in the zone substantially different from the direction of at least one electromagnetic field line produced by another of said field producing means at that point; and

means for sequentially applying electrical energy to each of said plurality of field producing means.

2. An apparatus according to claim 1 wherein said plurality of field producing means comprises:

a first means for producing a first electromagnetic field;

a second means for producing a second electromagnetic field each line of which has at substantially every point in the zone a direction substantially perpendicular to the direction of the line of the first field at that point.

3. An apparatus according to claim 2 wherein the first field producing means comprises a first long,

straight conductor, and wherein the second field producing means comprises at least one other long, straight conductor parallel to and co-extensive with the first conductor.

4. An apparatus according to claim 3 wherein the first field producing means comprises at least two long, straight parallel conductors.

S. An apparatus according to claim 4 wherein the second field producing means comprises at least two long, straight parallel conductors and the conductors of at least one of the first and second field producing means comprise parallel segments of a generally rectangular planar loop.

6. An apparatus according to claim 5 wherein said loop comprises a generally rectangular figure-8 coil and said parallel segments comprise the end and center segments of the figure-8 coil.

7. An apparatus according to claim 6 wherein the conductors of the other of said first and second field producing means also comprise parallel segments of a generally rectangular planar loop, the loops of the first and second field producing means being closely spaced and lying in substantially parallel planes.

8. An apparatus according to claim 2 wherein the first field producing means comprises a first long,

straight conductor and wherein the second field producing means comprises a second long, straight conductor orthogonal to and crossing the first conductor, the first and second conductors crossing far from their respective ends.

9. An apparatus according to claim 8 wherein at least one of the first and second field producing means further comprises at least one additional long, straight conductor positioned parallel to the conductor comprising the same field' producing means and lying in substantially the same plane as said first and second conductors.

10. An apparatus according to claim 9 wherein alternate ones of the conductors of said at least one field producing means are connected to said energizing means as a first group of conductors and wherein the other conductors of said atleast one field producing means are .connected to said energizing means as a second group of conductors, and wherein said energizing means energizes each of said groups in sequence. 11. An apparatus according to claim 8 wherein at least one of the first and the second field producing means comprises a center segment of a figure-8 coil. 12. An apparatus according to claim 2 wherein the first field producing means comprises a first substantially linear electromagnet; and wherein the second field producing means comprises a second substantially linear electromagnet disposed close to and substantially orthogonal to the first electromagnet, the first and second electromagnets crossing near their mid-points. 13. An apparatus according to claim 2 wherein the first field producing means comprises a substantially linear electromagnet and wherein the second field producing means comprises a sub- 6 stantially planar air core loop having a diameter approximately equal to the separation of the magtromagnet and wherein the electromagnet and air core loop are close-spaced with the periphery of the air core loop passing proximate the magnetic poles of the electromagnet.

14. An apparatus according to claim 2 further comprising:

third means for producing a third electromagnetic field the lines of which have a direction substantially mutually perpendicular to the directions of the first and second field lines at substantially every point in the interrogation zone.

15. An apparatus according to claim l4 wherein said first and third field producing means each comprise a generally rectangular figure-8 coil, each coil comprising a generally" rectangular figure-8 winding which forms two generally rectangular portions of nearly equal size, each portion having substantially parallel top and bottom lengths and a top and a bottom length of the two portions being substantially coincident to form a center segment of the figure-8 coil and the other of said top and bottom lengths of said portions forming end segments of the figure-8 coil, the figure-8 coils being oriented so that their center segments are orthogonal and cross near their mid-points; wherein said second field producing means comprises parallel segments of a generally rectangular 0 coil; wherein said figure-8 and 0 coils are closely spaced and lie in substantially parallel planes and wherein each ofthe parallel segments of the o coil are parallel to and lie between the center segment and one end segment of one figure- 8 coil.

16. An apparatus according to claim 15 wherein the means for sequentially applying energy comprises a source of electrical energy;

an energy transfer section coupled between each of said field producing means and the source of electrical energy, each energy transfer section including a switch responsive to a control signal to enable the transfer section to pass a pulse of energy from the energy source to the field producing means; and

a control signal generator for providing a control signal to each of the switches in a predetermined sequence.

17. An apparatus according to claim 16 wherein each field producing means is a coil having appreciable electrical inductance;

wherein said source of energy is a DC voltage;

and wherein each energy transfer section includes a capacitor and wherein the energy transfer section and field producing means form an inductivecapacitive circuit, the capacitor of the transfer section acting to store said pulse of energy upon actuation of said switch and to pass the energy to the field producing means as a damped pulse of ener- 18. An apparatus according to claim 17 wherein said inductive-capacitive circuit is an overdamped electrical circuit to produce an overdamped electromagnetic field pulse.

19. An apparatus according to claim 17 wherein said inductive-capacitive circuit is an underdamped circuit to produce an underdamped electromagnetic field pulse.

l7 18 20. A method of sequentially producing a plurality of rogation zone a plurality of pulses of electromagelectromagnetic fields within an interrogation zone netic field, the lines of each field being generally comprising the steps of curved and having respective directions at sub- Providing a plurality of field Producing means, each stantially every point in the zone substantially difof said means being positioned for producing in response to electrical energy supplied thereto an electromagnetic field the lines of which in an interrogation zone are generally curved and have a direction at substantially every point in the zone substantially different from the direction of at least 1 ferent from the direction of at least one other electromagnetic field line produced by another of said field producing means at that point. 21. A method according to claim 20 wherein said step of producing comprises producing the pulse one electromagnetic field line produced by sequenfze during a seqience petiod comprising: another of Said field producing means at that producing at least one continuous period of at least 1.0 point; and seconds dur ng which no field is produced; and, sequentially applying electrical energy to each of producing said pulse sequence during an interval of not said plurality of field producing means; longer than seconds thereby producing as a pulse sequence in the inter- 

1. An apparatus for sequentially producing a plurality of electromagnetic fields within an interrogation zone comprising: a plurality of field producing means, each being positioned for producing in response to electrical energy supplied thereto an electromagnetic field the lines of which in said interrogation zone are generally curved and have a direction at substantially every point in the zone substantially different from the direction of at least one electromagnetic field line produced by another of said field producing means at that point; and means for sequentially applying electrical energy to each of said plurality of field producing means.
 2. An apparatus according to claim 1 wherein said plurality of field producing means comprises: a first means for producing a first electromagnetic field; a second means for producing a second electromagnetic field each line of which has at substantially every point in the zone a direction substantially perpendicular to the direction of the line of the first field at that point.
 3. An apparatus according to claim 2 wherein the first field producing means comprises a first long, straight conductor, and wherein the second field producing means comprises at least one other long, straight conductor parallel to and co-extensive with the first conductor.
 4. An apparatus according to claim 3 wherein the first field producing means comprises at least two long, straight parallel conductors.
 5. An apparatus according to claim 4 wherein the second field producing means comprises at least two long, straight parallel conductors and the conductors of at least one of the first and second field producing means comprise parallel segments of a generally rectangular planar loop.
 6. An apparatus according to claim 5 wherein said loop comprises a generally rectangular figure-8 coil and said parallel segments comprise the end and center segments of the figure-8 coil.
 7. An apparatus according to claim 6 wherein the conductors of the other of said first and second field producing means also comprise parallel segments of a generally rectangular planar loop, the loops of the first and second field producing means being closely spaced and lying in substantially parallel planes.
 8. An apparatus according to claim 2 wherein the first field producing means comprises a first long, straight conductor and wherein the second field producing means comprises a second long, straight conductor orthogonal to and crossing the first conductor, the first and second conductors crossing far from their respective ends.
 9. An apparatus according to claim 8 wherein at least one of the first and second field producing means further comprises at least one additional long, straight conductor positioned paralLel to the conductor comprising the same field producing means and lying in substantially the same plane as said first and second conductors.
 10. An apparatus according to claim 9 wherein alternate ones of the conductors of said at least one field producing means are connected to said energizing means as a first group of conductors and wherein the other conductors of said at least one field producing means are connected to said energizing means as a second group of conductors, and wherein said energizing means energizes each of said groups in sequence.
 11. An apparatus according to claim 8 wherein at least one of the first and the second field producing means comprises a center segment of a figure-8 coil.
 12. An apparatus according to claim 2 wherein the first field producing means comprises a first substantially linear electromagnet; and wherein the second field producing means comprises a second substantially linear electromagnet disposed close to and substantially orthogonal to the first electromagnet, the first and second electromagnets crossing near their mid-points.
 13. An apparatus according to claim 2 wherein the first field producing means comprises a substantially linear electromagnet and wherein the second field producing means comprises a substantially planar air core loop having a diameter approximately equal to the separation of the magnetic poles of said first substantially linear electromagnet and wherein the electromagnet and air core loop are close-spaced with the periphery of the air core loop passing proximate the magnetic poles of the electromagnet.
 14. An apparatus according to claim 2 further comprising: third means for producing a third electromagnetic field the lines of which have a direction substantially mutually perpendicular to the directions of the first and second field lines at substantially every point in the interrogation zone.
 15. An apparatus according to claim 14 wherein said first and third field producing means each comprise a generally rectangular figure-8 coil, each coil comprising a generally rectangular figure-8 winding which forms two generally rectangular portions of nearly equal size, each portion having substantially parallel top and bottom lengths and a top and a bottom length of the two portions being substantially coincident to form a center segment of the figure-8 coil and the other of said top and bottom lengths of said portions forming end segments of the figure-8 coil, the figure-8 coils being oriented so that their center segments are orthogonal and cross near their mid-points; wherein said second field producing means comprises parallel segments of a generally rectangular o coil; wherein said figure-8 and o coils are closely spaced and lie in substantially parallel planes and wherein each of the parallel segments of the o coil are parallel to and lie between the center segment and one end segment of one figure-8 coil.
 16. An apparatus according to claim 15 wherein the means for sequentially applying energy comprises a source of electrical energy; an energy transfer section coupled between each of said field producing means and the source of electrical energy, each energy transfer section including a switch responsive to a control signal to enable the transfer section to pass a pulse of energy from the energy source to the field producing means; and a control signal generator for providing a control signal to each of the switches in a predetermined sequence.
 17. An apparatus according to claim 16 wherein each field producing means is a coil having appreciable electrical inductance; wherein said source of energy is a DC voltage; and wherein each energy transfer section includes a capacitor and wherein the energy transfer section and field producing means form an inductive-capacitive circuit, the capacitor of the transfer section acting to store said pulse of energy upon actuatiOn of said switch and to pass the energy to the field producing means as a damped pulse of energy.
 18. An apparatus according to claim 17 wherein said inductive-capacitive circuit is an overdamped electrical circuit to produce an overdamped electromagnetic field pulse.
 19. An apparatus according to claim 17 wherein said inductive-capacitive circuit is an underdamped circuit to produce an underdamped electromagnetic field pulse.
 20. A method of sequentially producing a plurality of electromagnetic fields within an interrogation zone comprising the steps of providing a plurality of field producing means, each of said means being positioned for producing in response to electrical energy supplied thereto an electromagnetic field the lines of which in an interrogation zone are generally curved and have a direction at substantially every point in the zone substantially different from the direction of at least one electromagnetic field line produced by another of said field producing means at that point; and sequentially applying electrical energy to each of said plurality of field producing means; thereby producing as a pulse sequence in the interrogation zone a plurality of pulses of electromagnetic field, the lines of each field being generally curved and having respective directions at substantially every point in the zone substantially different from the direction of at least one other electromagnetic field line produced by another of said field producing means at that point.
 21. A method according to claim 20 wherein said step of producing comprises producing the pulse sequence during a sequence period comprising: producing at least one continuous period of at least 1.0 seconds during which no field is produced; and, producing said pulse sequence during an interval of not longer than 0.4 seconds. 